[tz] OpenJDK/CLDR/ICU/Joda issues with Ireland change

Yoshito Umaoka yoshito_umaoka at us.ibm.com
Wed Jan 24 21:17:15 UTC 2018

> > CLDR does not determine offsets.
> Stephen Colebourne claimed that CLDR determines whether to use the 
> standard or daylight time strings by comparing the "raw offset" 
> (presumably meaning "the offset during standard time") with the 
> "actual offset" (presumably meaning "the offset during daylight 
> savings time").
> Therefore, it *must* know those offsets, otherwise it cannot compare 
> So let me rephrase the question:
>    How does CLDR obtain those offsets?

CLDR only maintains names for each type in XML format.

CLDR XML (or JSON) data is consumed by other projects such as ICU and 
Java, and these external projects know those offsets.
CLDR only specifies daylight saving time name used for Europe/Dublin is 
"Irish Standard Time".
ICU/Java imports zoneinfo from tz database, and obtain offset at a given 
time, then decide whether it's in standard time or daylight time. When 
ICU/Java display a date with time zone, it just load the display name 
separately imported from CLDR - with time zone ID Europe/Dublin. (The 
logic explained here is a little bit simplified actually. CLDR also 
provide historic name mapping changes, and ICU utilizes the data as well.)

Because the runtime code such as ICU and Java detects offsets, and 
separately imported data for CLDR maintain display names, they must be 
updated at the same time.


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